1. Field of the Invention
This invention relates to a recording method which utilizes chemical or physical changes of a diacetylene derivative compound to conduct recording, as well as to a recording medium used in said recording method.
2. Related Background Art
Various recording media using an organic compound as a recording layer have conventionally been known.
For example, optical recording media employing an organic compound in the form of a thin film as a recording layer are disclosed, for example, in Japanese Patent Laid-Open Publication Nos. 16948/1981 and 125246/1983. Each of these publications discloses a laser recording medium which uses an organic dye as a recording layer and can conduct recording and reproduction using a laser beam. The recording medium disclosed in Japanese Patent Laid-Open No. 125246/1983, in particular, uses as a recording layer a thin film of a cyanine dye represented by the following general formula (I). ##STR1## A solution of a cyanine dye represented by the formula (I) is coated on a plastic substrate in thickness of 1,000 .ANG. or below (e.g. 300 .ANG.) using a rotary coater or the like, to form a thin film. In this case, if the distribution and orientation of the dye molecules in the film are at random, light will be scattered in the film on irradiation of the film, and thus a chemical reaction degree varies microscopically at each light exposure. Hence, the molecules in the film are desired to have uniform distribution and uniform orientation, and further the film is required to be made as thin as possible to obtain high density recording. However, the film thickness has been limited to about 300 .ANG. when the film is prepared according to a coating method and further in this method, the random distribution and random orientation of molecules in the film have been unavoidable.
It is known that films formed by coating a solution of a diacetylene derivative compound on a carrier and drying the solution have the following properties.
(1) Such a film is colorless in the original state but, when exposed to ultraviolet rays to cause a certain degree of polymerization, is changed to a first state giving a blue color. This first state of blue color can not be returned to the colorless original state even after the exposure to ultraviolet rays is stopped. Therefore, the change from the original state to the first state due to exposure to ultraviolet rays is irreversible.
(2) The film in the first state, when heated to about 50.degree. C. by applying a thermal energy, is changed to a second state of red color. This second state can not be returned to the first state of blue color even after the heating is stopped and the film is cooled. Therefore, the change from the first state to the second state due to a heat is also irreversible.
(3) The film in the second state, when heated to about 300.degree. C. by applying a thermal energy, is changed to a third state of yellow color. This third state can be returned to the second state when the heating is stopped and the film is cooled. Therefore, the change between the second state and the third state is reversible.
Thus it is known that a colorless and transparent film made from a diacetylene derivative compound, when exposed to ultraviolet rays, causes a polymerization reaction and turns blue.
This color development reaction has been considered to be caused only by ultraviolet rays or gamma-ray and not by a light of relatively high wavelength. [G. Wegner, "Chemistry and Physics of One-Dimensional Metals", Plenum (1977), p. 297]
Further, the film made from a diacetylene derivative compound has a drawback that the film, when allowed to stand indoors, is affected by an outdoor light, particularly, ultraviolet rays to cause deterioration with time resulting poor storability.